In the drilling and completion of wells for producing hydrocarbon fluids, for example, the wellbore is often required to be extended through one or more earth formation zones or intervals which are highly fractured, rubblized or otherwise highly conductive of fluids. In recent years efforts to develop hydrocarbon reserves have resulted in drilling highly deviated or so-called horizontal wellbores wherein a portion of the wellbore from which fluids are to be produced extends through a formation zone that may have several spaced-apart, generally vertically extending fractures. These fractured areas are typically oil bearing and are desired to be produced once the well is completed.
However, in the drilling process, as the wellbore penetrates each fracture or a highly fractured or rubblized area, substantial quantities of drilling fluid may be lost to the formation. It is not unusual for drilling fluid losses to exceed 5,000 to 10,000 barrels per well, representing substantial costs to the drilling operation. The addition of casing strings and conventional cementing operations to such wells only results in substantial additional costs. Moreover, carrying out conventional cementing operations or the injection of conventional fluid loss control materials may cause substantial loss of conductivity of the formation interval to which the drilling fluid has been lost. However, quite often this interval is also the one from which production of formation fluids is desired once the well is completed. Accordingly, there has been a continuing need to develop a method and means for preventing fluid loss to highly conductive earth formation zones during the drilling and completion of wells therein, including, in particular, deviated and generally horizontally extending wells which may penetrate multiple fractures in a fluid producing zone.
It has been suggested to provide improved support for well casings in unconsolidated formations and/or non-vertical well bores and to prevent the migration of particulate materials into well casings from unconsolidated formations and/or from non-vertical wellbores and to prevent wellbore collapse in unconsolidated formations and/or non-vertical wellbores by cementing a casing or liner in the wellbore using a foamed permeable cement composition injected into the wellbore annulus between the casing and the wellbore wall. U.S. Pat. No. 5,339,902 to Harris et al., assigned to Halliburton Company, Duncan, Okla. and to the assignee of the present invention, describes a method of cementing a well casing or liner in a wellbore by placing a foamed cement slurry composition in the wellbore annulus. The cement is allowed to harden and then drilling is carried out further through the hardened cement composition which remains in a cased or uncased portion of the wellbore adjacent to the annular area in which the cement composition has been placed to stabilize the wellbore and to secure the casing in place in the wellbore. The foamed cement composition is provided by dry mixing a cementitious material with any dry additives which are to be included in the composition, mixing the resulting dry blend of materials with water and any liquid additive to form a slurry, then pumping the slurry into the wellbore and injecting gas into the cement composition as it flows into the well.
The Harris et al. patent suggests pumping the foamed cement slurry through a tubing string disposed within a slotted or perforated well casing and wherein spaced apart fluid flow blocking devices, such as packers, are positioned on opposite sides of the slotted or perforated casing portion. The Harris et al. patent also suggests that the foamed cement composition may be pumped into an uncased portion of the well, allowed to harden and then drilling continued through the hardened cement material along at least a portion of the uncased well. The resulting permeable cement structure allows the free flow of oil and gas or other formation fluids into the well but prevents the substantial migration of formation particulates into the well. Still further, the above-mentioned patent suggests that the cement permeability may be enhanced by treating with hydrochloric and/or hydrofluoric acid or other matrix material treatments to increase the cement pore size and permeability.
More important, perhaps, than the elimination of wellbore collapse or production of fines from unconsolidated formations, is the first mentioned problem of eliminating substantial fluid loss during drilling operations and the resulting damage to fluid producing fractures, rubblized formation intervals and other highly conductive formations. Whereas the production of fines from an unconsolidated formation or the collapse of a wellbore can be remedied, the unwanted plugging of a producing formation interval, such as formation intervals provided by spaced apart vertical fractures, can result in permanent damage and loss of fluid production from the interval. It is to these ends that the present invention has been developed.